The description of "Medusa head" colonies on solid agar is most characteristic of which organism?

The description of "Medusa head" colonies on solid agar is most characteristic of which organism?

• A. Bacillus anthracis
• B. Enterococcus faecalis
• C. Staphylococcus saprophyticus
• D. Streptococcus agalactiae

Answer: (A)

The main idea is recognizing a distinctive colony morphology that signals Bacillus anthracis. “Medusa head” describes a colony with radiating, hair-like projections around the main colony, giving a fringe or feathery edge. This fringe pattern on solid media, such as blood agar, is a classic description for Bacillus anthracis, a large Gram-positive rod that is nonmotile and encapsulated. The other organisms listed are cocci (and different in their colony appearance), so they don’t produce the characteristic medusa-like fringe. Thus, the description points most strongly to Bacillus anthracis.

The main idea is recognizing a distinctive colony morphology that signals Bacillus anthracis. “Medusa head” describes a colony with radiating, hair-like projections around the main colony, giving a fringe or feathery edge. This fringe pattern on solid media, such as blood agar, is a classic description for Bacillus anthracis, a large Gram-positive rod that is nonmotile and encapsulated. The other organisms listed are cocci (and different in their colony appearance), so they don’t produce the characteristic medusa-like fringe. Thus, the description points most strongly to Bacillus anthracis.